The most remarkable characteristics of mobile communication are complicacy and time-dependency of channels. In a coherent reception scheme, it is necessary to estimate and measure a channel at the receiver side, and then coherent detection is implemented for a signal by means of the received channel response. In addition to its application in signal detection, the channel estimation also plays an important and basic role in relevant subsystems such as of physical layer measurement, intelligent antenna, fast control, switching, radio resource management, etc.
In a time-slot CDMA (Code Division Multiple Access) cellular mobile communication system, a channel estimation code and a data portion of a transmission are transmitted in a time multiplex manner and different cells are in time slot synchronization.
For example, in the TD-SCDMA (Time Division Synchronous CDMA, i.e., 3GPP 1.28 Mcps TDD) system, the structure of a burst signal in a service time slot is shown in FIG. 1.
In the middle of the burst signal, a midamble (also referred to as a channel estimation code) is provided to perform the channel estimation. The data blocks on both sides of the midamble are provided to transmit the service data.
The channel estimation code (midamble) described above is generated in the following ways.
For a same time slot of a cell, a basic midamble is given as a basic code, and different cyclic shift versions of the same basic code are taken by different users as his/her channel estimation code. K specific midambles mP(k) (k−1, . . . , K) deduced from one single basic code mP constitute a set of midambles, which may be referred to as a code-set for short.
In the TD-SCDMA system, the period of a basic code is P=128 and the length of a deduced midamble is Lm=128+6. What is used for channel estimation in the time-slot CDMA system is called a Steiner estimator. For detailed content, reference may be made to B. Steiner, and P. W. Baier, “Low Cost Channel Estimation in the Uplink Receiver of CDMA Mobile Radio Systems,” FRE QUENZE, 47 (1993) 11-12. The specific estimation process is as follows:
It is assumed that the channel response of each user is h(k):h(k)=(h1(k),h2(k), . . . , hW(k))T  (1)
Where the window length W is used to indicate the time length of the channel response and the channel response vector is represented by the values on W taps separated by chips. In this case, the length of a midamble response signal should be Lm+W−1. In view of successive transmission of the midamble and the transmitted data portion, the first W−1 values of the midamble response signal are affected by the preceding data block(s) and the last W−1 values are overlapped with the subsequent data block(s).
Taking P chips in the middle as observed values and the vector is expressed as:emid=(e1,e2, . . . , eP)  (2)
According to the characteristics of the generation, within one code-set, of the midambles of multiple users, the response signal to the midamble received by the receiver may be expressed as:emid=Gh+n  (3)Where n indicates noise and interference, and h indicates the total channel response vector:h=(h(1)T,h(2)T, . . . , h(K)T)T  (4)The matrix G is a cyclic matrix:G=(circle(g1,g2, . . . , gP))T  (5)
Where g=(g1, g2, . . . , gP) is the first column of the matrix C, determined by the basic code which generates the code-set.
The task of the channel estimation is to obtain an estimation value of the channel response h by solving the equation (3) with h as the unknown variable. The following can be obtained by using the maximum likelihood rule and implementing a simplification to some extent:ĥ=G−1emid  (6)
Since the matrix G is a cyclic matrix, the actual operation may be implemented through 2-Dimension Discrete Fourier Transform (DFT) and Inverse 2-Dimension Discrete Fourier Transform (IDFT).
                              h          ^                =                  IDFT          ⁡                      (                                          DFT                ⁡                                  (                                      e                    mid                                    )                                                            DFT                ⁡                                  (                  g                  )                                                      )                                              (        7        )            
DFT(g) in the equation may be obtained by off-line calculation in advance.
By using this scheme of channel estimation, the channel estimation results of multiple users within a code-set are obtained with reduced calculation cost, and the interference among midambles of multiple users belonging to the same basic code is suppressed. This scheme of channel estimation aims to perform channel estimation with respect to the midambles of multiple users within a single code-set belonging to a single basic code, that is to say, the method is a uni-code-set channel estimation method.
In the uni-code-set channel estimation method, all the other signals, superposed at the same time, than the response to the signal corresponding to the concerned code-set, are treated as white Gaussian noises. Therefore, when the ratio of signal to interference and noise is low, the performance of the uni-code-set channel estimation method can not satisfy requirements. In addition, for a time-slot CDMA system with synchronously operating cells, the communication signals of the users in a neighbor cell who is located adjacent to the border between the neighbor cell and the concerned cell interfere greatly with the concerned cell, and the signals of a neighbor cell with the same frequency are in time slot synchronization with the signals of the concerned cell. That is to say, the interferences that the channel estimation code of the concerned cell suffers from neighbor cells having the same frequency mainly come from the responses to the channel estimation code signals in other code-sets synchronous with the concerned cell. In addition, in some other applications, it is also possible that the signals, with similar powers, responding to the channel estimation codes of multiple code-sets will overlap with each other. For example, when taking multiple antennas to perform reception, the performance of the uni-code-set channel estimation method is deteriorated in performance, thus not only limiting the improvement of the reception performance of the multi-antenna system, but also introducing performance deterioration in relevant subsystems of the multi-antenna system, such as subsystems of the forming transmission (beamforming transmission), physical layer measurement, synchronization and power control, etc. In this case, the uni-code-set channel estimation method is difficult to satisfy the requirements of the system with respect to the channel estimation performance.
Therefore, in Chinese Patent Application No. 03100670.1, a multi-code-set channel estimation method in a time-slot CDMA system is proposed. According to the method, a multi-code-set joint channel estimation is implemented based on a limited number of decisions of time positions.
In a practical multi-code-set channel estimation, on one hand the performance of the multi-code-set channel estimation is difficult to approach the performance of an ideal channel estimation because of the effect of the unwanted cross correlation among the channel estimation codes of various code-sets; on the other hand, it is complicated in calculation for better channel estimation performance and the cost of implementation is high.